Polyquinoxalines are prepared by polycondensation of tetraketones or biketonic dialdehydes with tetramines according to the following scheme: ##STR3## with n.gtoreq.10 and R=H or phenyl radical,
G=carbon-carbon bond or --O--, --S--, --SO.sub.2 --, --CO--. ##STR4## PA0 S=solubility coefficient of gas in the membrane. PA0 D=diffusion coefficient of gas in the membrane.
Such polycondensation is generally carried out in solution (see for example: G. de GAUDEMARIS, B. SILLION, J. PREVE, Bull. Soc. Chem. (1964) 1793--J. K. STILLE, J. R. WILLIAMSON, J. Polym. Sci. /A/ 2. 3867 (1964) and P. M. HERGENROTHER, Polymer Eng. Sci. 16 (No. 5) 303 (1976)).
The polymers of the polyquinoxaline family are characterized by good chemical and thermal stability which makes them particularly capable of resisting drastic conditions of use (oxidation, extreme pH and the like). Their solubility in certain chlorinated hydrocarbons and m-cresol permits formation of films and membranes therefrom.
Membranes such as those described in French Pat. No. 2 392 697 based on such polymers are particularly known but these are microporous membranes for ultrafiltration, such process implying separation by a simple flow-through process which has nothing to do with processes used in gaseous permeation separation. As a matter of fact, the passage of a gas through a gaseous permeation membrane occurs according to a dissolution and diffusion mechanism following Fick's laws.
Such process implies gas dissolution in the active portion of the membranous material, for example, at the upstream face of the membrane, then diffusion of the dissolved molecules of the upstream face towards the downstream face under the effect of the pressure differential between the two faces and finally evaporation of the gas molecules on the downstream face of the membrane.
Due to this, the permeability coefficient P of a gas through the membrane is defined as being the product of two coefficients: P=S. D.
In order for the membrane to be able to separate two gases it is necessary that the permeability coefficients of the two gases differ from one another.
Transportation of gas according to this mechanism is different from that occurring by flowing through microporous media. The passage of a gas through a microporous membrane occurs according to KNUDSEN's law if the average free travel of the gas is higher than the pore diameter. In this case, the permeability coefficients P.sub.1 and P.sub.2 of two gases of molecular mass M.sub.1 and M.sub.2 correspond to the following ratio: ##EQU1##
In order to have separation of the two gases it is necessary that their molecular masses be clearly different.
Thus when it is desired to efficiently separate gases of similar molecular masses, for example CO.sub.2, H.sub.2 O and CH.sub.4, it is necessary that the passage of such gases should not occur through a simply microporous membrane since otherwise the separation factor between such gases as taken by pairs will be according to the ratio of the square root of their molecular masses, i.e.: ##EQU2## but this is too small a separation to present any practical interest.
In order for the gaseous permeation phenomenon to occur fully it is significant for the membrane to present a dense layer on one face. By "dense" layer it is meant a layer without any pores permitting direct through-passage of the gaseous molecules by simple flowing through under the effect of pressure.
Such pores will have to be of a diameter much lower than 1 nm, preferably lower than 0.5 nm.
Since the rate of diffusion of gas molecules through such a zone is low it is desirable that the thickness of such zone should be as low as possible and represent only a fraction of the total thickness of the membrane; the other portion of the membrane preferably comprises a microporous structure zone so that the flow of gas therethrough is not slowed down.
Furthermore, as for most of the filmogenic polymers, polyquinoxalines, of a sufficiently high molecular mass, permit preparation of membranes or films having good mechanical strength. Such films or membranes, dense and of a homogeneous structure, can be prepared by pouring by means of an applicator the solution of polyquinoxaline of sufficient viscosity onto a plane carrier and by completely evaporating the solvent. To completely remove the solvent it might be useful to effect such evaporation under vacuum and at temperatures higher than 100.degree. C. Under these conditions, films or membranes are obtained, having an average diameter of pores lower than 10 .ANG..
To determine permeability of this type of polymer relative to different gases the Applicants have prepared polyphenylquinoxaline films of a thickness of 10 to 20 microns by complete evaporation of the solvent.
The measurements of gaseous permeability have shown that surprisingly such films were more permeable to water vapor than to other gases. The Table 1 hereinbelow shows the values of permeabilities obtained for different gases at 20.degree. C. and under a pressure of 3 bars (except for water vapor the permeability of which was measured at 50.degree. C. and a relative pressure of 0.56 to 60), as well as the separation factors corresponding to gas permeability ratios. The permeabilities are expressed in units cm.sup.4 /cm.sup.2.sec.cm. Hg representing the volume as measured in cm.sup.3 having passed through a membrane of 1 cm.sup.2 of useful area and 1 cm of thickness in 1 second under a pressure differential of 1 cm mercury.
The obtained values can be considered as being close to the intrinsic permeabilities of the polyphenylquinoalines since the films were prepared under experimental conditions which prevent formation of pores. Due to this, one may consider that the passage of gases through the film occurs according to a dissolution and diffusion mechanism which follows Fick's laws. Only the presence of any defects in the film might be the cause of gas passage by flowing through the pores.
The comparison of the intrinsic permeabilities of such films with those of films prepared from other polymers (Table 2 hereinbelow) show that only cellulose acetate possesses a separation factor between water vapor and methane higher than that of polyphenylquinoxaline for an equivalent CH.sub.4 -permeability, but on the other hand, a lower separation factor between the carbon dioxide and methane. Moreover, polyphenylquinoxaline presents much higher chemical and thermal stability than cellulose acetate. For these reasons the polymers of the polyquinoxaline family are of a special interest for separation of water vapor and carbon dioxide from gaseous hydrocarbons.
TABLE 1 __________________________________________________________________________ Intrinsic permeabilities P and factors of separation of homogeneous films in polyphenylquinoxaline in respect to different gas. ##STR5## Measurement temperature: 20.degree. C. P .multidot. 10.sup.10 Separation factors O.sub.2 CO.sub.2 N.sub.2 CH.sub.4 He H.sub.2 O + H.sub.2 O/CH.sub.4 CO.sub.2 /O.sub.2 CO.sub.2 /CH.sub.4 H.sub.2 O/O.sub.2 __________________________________________________________________________ 1.0 6.0 0.3 0.3 7.4 1800 6000 6 20 1800 __________________________________________________________________________ ##STR6##
TABLE 2 ______________________________________ Intrinsic permeabilities and separation factors of dense films in respect to water vapor, carbon dioxide and methane. ##STR7## Nature of P .multidot. 10.sup.10 Separation factor film CH.sub.4 CO.sub.2 H.sub.2 O H.sub.2 O/CH.sub.4 CO.sub.2 /CH.sub.4 ______________________________________ Poly- 590 3200 40000 68 5.4 dimethyl- siloxane* Ethyl 6 40 24000 4000 6.7 cellulose* Poly- 3.6 8 1400 389 2.2 carbonate* Vinyl 2 3.7 590 295 1.8 poly- chloride* Polysulfone 1.7 4.4 1200 705 2.6 TEFLON 1.4 1.7 33 24 1.2 FEP* Cellulose 0.3 2.4 10000 33000 8 acetate* Polyphenyl- 0.3 6 1800 6000 20 quinoxaline Polyethyl- 0.006 0.15 175 29170 25 enetere- phthalate ______________________________________ *Values published by W. PUSCH & A. WALCH in Angew. Chem. Int. Ed. Engl. 2 (1982) 660-685.